The present invention relates to a multilayer printed circuit board and more specifically to a transition between a symmetric stripline and microstrip in the cavity of such a board.
Many different sorts of multilayer printed circuit boards are known to the art. LTCC (Low Temperature Co-fired Ceramic) will be used hereinafter as an example, although it will be understood that the invention can also be applied in other types of multilayer printed circuit boards.
Briefly, multilayer printed circuit boards are manufactured in the following way. There is obtained on the basis of a printed circuit board design a drawing that contains necessary information, such as the number of layers, the appearance and dimensions of the patterns on the various layers, the locations at which different layers shall contact one another, and so on.
Each layer per se is rolled out from a ceramic mass to a predetermined thickness on a plastic film; this is a so-called tape. Different patterns are punched from these tapes in accordance with the design, these including the outer edges of the board, the marks that are later used to match the layers together, and holes for binding different layers together with so-called vias.
Subsequent to configuring the layers, the via holes are filled with a suitable conductive material. The patterns are then printed on each of the layers. A common method in this respect is to use screen-printing to correctly position the conductors. These conductors may consist of gold, silver or some other suitable conductive material. When the patterns are in place, the various layers are placed one upon the other until all layers are in position.
The whole of the printed circuit board is then placed under pressure, inserted into an oven and baked immediately (Co-fired) at a relatively low temperature, 700-800 degrees centigrade (Low Temperature), wherewith the ceramic mass is sintered and transformed to a ceramic. Subsequent to this curing or hardening process, it is usual to speak of layers instead of tapes.
In the case of applications for high frequency signals, particularly within the microwave field, it is not always possible to use traditional conductors, since this would result in unacceptable losses and disturbances. A normal requirement in the case of microwave signals is the presence of an earth plane above or beneath a conductor, this earth plane following the conductor. When a conductor only has an earth plane on one side it is called a microstrip. These strips are normally arranged so that they have the printed circuit board on one side and air or another dielectric on the other side. In other cases, it is desirable that the conductor is surrounded by both an upper and a lower earth plane, this conductor then being called a stripline. When the distances between a stripline and the earth planes are the same on both sides of the conductor, it is said that the stripline is symmetrical. When the distances are different, an asymmetric stripline is obtained. Although symmetric striplines are he most common, there are occasions when an asymmetric stripline is preferred. One advantage afforded by striplines is that radiation from the conductors is small when, e.g., transmitting signals in the microwave range in so-called stripline-mode, which is one reason why such signals are often transmitted in this way. Microstrips and striplines can be easily provided in multilayer printed circuit boards, and are consequently often used to this end. In order to enable conductors to be surrounded by earth planes, conductor planes and earth planes are normally disposed alternately in the printed circuit board.
It is possible to mount chips, for instance an MMIC (Monolithic Microwave Integrated Circuit), directly in a multilayer printed circuit board. This is achieved by placing the chip on the earth plane in a cavity and connecting the chip to the nearest signal carrying layers with the aid of so-called bonding wires. This is shown in FIG. 1, in which the chip has been connected to a pair of microstrips in a known manner. The transition between the symmetric striplines and the microstrips, however, is not fully satisfactory, since the electric field in the strip line is tied just as strongly to the upper earth plane as to the lower earth plane. This means that when the upper earth plane suddenly disappears, it becomes xe2x80x9cheavy goingxe2x80x9d for the conductor and thus results in a poor match.
This matching problem does not apply to signals that have sufficiently low frequencies. On the other hand, the problem does arise in the case of high frequency signals, for instance RF signals. The reason why these signals are nevertheless still transmitted in so-called stripline mode is because it reduces radiation from the conductors, among other things.
The present invention addresses the problem of improving matching in a transition between a stripline and a microstrip in a cavity, preferably in a multilayer printed circuit board that has chips mounted therein.
One object of the present invention is thus to provide a transition between stripline and microstrip in a cavity such that good matching will be obtained.
In brief, the present invention relates to an arrangement in which microstrip and stripline are asymmetrical instead of symmetrical. The electric field in the stripline will thus essentially be undisturbed downstream of the transition to the microstrip, since the field is tied to the adjacent earth plane.
The inventive arrangement is characterised by the features set forth in the accompanying Claim 1.
One advantage afforded by the solution to said problem is that transitions between stripline and microstrip in a cavity can be made simpler and with good matching. This enables multilayer printed circuit boards which have components mounted therein to be made more effective.
The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawing.